**INTRODUCTION OF MOTION AND MEASUREMENT** : In daily life, the importance of measurement is well known. Whenever we have to bring fruits, vegetables etc. from the market, the seller measures their quantity using beam balance and we have to pay him accordingly. The tailor needs exact measurement of our body to stitch our dresses. We always have an approximation as to how much time will be spent on a journey for our convenience. Thus, we can conclude that measurement is an essential part of everyday life.

**Measurement** : It is the determination of the dimensions, quantity or capacity of an item.

**Physical quantity** : Any quantity that we study in physics ( e.g. length, mass and time) is called physical quantity. To measure physical quantities such as mass, time and length, we use units such as kilogram, second, and metre.

**Unit **: A physical quantity (such as length) has to be measured with respect to some known quantity. This known quantity is called a unit.

**HISTORY OF LENGTH MEASUREMENT**

In ancient times, different body parts were used to measure length. The distance between the tip of the thumb and the tip of the little finger of a fully stretched hand was termed a hand span. The distance between the tip of the middle finger and the elbow was termed a cubit. Foot, stride, fathom and yard are some more units based on the length of body parts. However, it was soon realized that these units were not very reliable as the length of body parts can vary from person to person.

We need standard units of length and other physical quantities to obtain the same value for a measurement.

However, people across the world might adopt different standard units. For example, gram and pound are units of mass used by people of different countries. To solve this problem, we need a set of standard units which is acceptable throughout the world.

**STANDARD SYSTEM OF MEASUREMENT**

Standard units are those that have a fixed quantity and, therefore, do not vary from person to person and place to place. For example, the metric system created by the French in 1790, is a standard set of units.

Adopting standard units of measurement solves only half the problem. People in different countries may be using different sets of standard units for measurement. For example, gram and pound set of units. The adoption of SI units has made it easier for scientists of different countries to communicate their results to one another.

**Mind-Bender : Why we need fixed or standard units of measurement?**

The SI system of units was adopted in 1960 by the General Conference of Weights and Measures. SI is the short form of **Internationale Systeme d’units**. The SI unit of length is the metre. Other common standard units of length are inch, millimetre, centimetre, kilometre and so on. SI unit of mass and time are kilogram and second respectively.

Depending on the size of the object we wish to measure, we have to choose an appropriate unit. For example, we use metres to measure the length of a piece of cloth, kilometres to measure the distance from one place to another, millimetres to measure the thickness of the hair, and so on. Units of length can be interconverted by multiplying or dividing their values by 10, 100, 1000, and so on.

A commonly used unit for measuring large distances is the kilometre (represented as km). Commonly used units for measuring small distances are the centimetre (cm) and millimetre (mm).

1000 m = 1 km

100 cm = 1m

1000 mm = 1 m

For measuring lengths smaller than the metre, its sub-multiples are used. For lengths greater than the metre, its multiples are used.